Using magneto resistive (MR) sensors for magnetic field sensing as replacements for the Hall sensors currently being used in commercial products, has the advantage of much higher sensitivity. By using state-of-the-art magnetic tunnel junction (MTJ) sensors, sensitivities an order of magnitude greater than that of Hall sensors can be achieved.
The use of MR devices as current sensors in circuits to which they are not connected (by sensing the magnetic fields associated with these circuits) is disclosed in the Prior Art[1-2]. FIG. 1A shows the general structure of a closed-loop current field sensor. It has three key parts—MR sensor 1, current carrying electric line 3, and field-canceling electric line 4. The current in electric line 3, which is situated beneath the substrate 2 where the MR sensor sits, is the physical quantity to be measured by the MR sensor. When current 5 is flowing through line 3, it generates a magnetic field at MR sensor 1 as represented by the field line 6. This field will rotate the magnetization of the sensing layer of 1 which results in a resistance change across MR sensor 1.
During a closed-loop operation, a second current 7 runs through the field-cancelling line 4 adjacent to MR sensor 1 and produces a field 8 in MR sensor 1 which offsets field 6 in MR sensor 1 generated by current 5. When the field 8 generated by line 4 in sensor 1 has been calibrated, the current 7 that is required to completely offset the field 6 generated by 5 and produces an effective zero total field in MR sensor 1, i.e. brings the resistance of sensor 1 to its zero field value, serves as a measurement of the current value 5 running in line 3.
Two other prior art sensing schemes are illustrated in FIGS. 1B and FIG. 1C. FIG. 1B is an open-loop configuration wherein a current source powers MR sensor 9 and the voltage across the sensor is compared to a reference voltage Vref, which is set to equal sensor voltage 9 in the absence of an external magnetic field. Thus, the output voltage Vout is proportional to the resistance change of the sensor 9. If the sensor 9 resistance has a reasonably linear response to the external field, Vout can then be a measurement of the external magnetic field that is being measured by sensor 9.
FIG. 1C shows another scheme, where two MR sensors, 9 and 10, that have similar resistance and sensitivity characteristics, are connected in serial and powered by voltage source VCC. However, the MR sensors are arranged to be in opposition so that, when exposed to the same magnetic field, the resistance of one increases while that of the other sensor decreases.
Thus, for the FIG. 1C configuration, whenever there is a magnetic field present, the voltage at the connection point of the two MR sensors will deviate from VCC/2. When Vref is set as VCC/2, Vout changes accordingly. The FIGS. 1B and 1C structures are generally applicable to field sensing applications but 1C and 1A can be readily combined whereby an accurate measurement of underlying electric current 5 is obtained from current value 7 when Vout (of FIG. 1C) is offset to zero.
What limits the 1B and 1C circuits, as implemented in the prior art, is that these schemes use the resistance of the MR sensors directly. The FIG. 1B open-loop structure is sensitive to the absolute value of the resistance fluctuation, so resistance shifts during operation due, for example, to thermal, electrical or mechanical stresses, will directly affect the accuracy of the measurement.
The accuracy of the FIG. 1C scheme, though not affected by the absolute resistance shifts of the two MR sensors, is affected by relative resistance shifts between the two sensors, i.e. if the sensors' resistances do not change to the same degree when undergoing the same kind of stress.
FIG. 2 shows how the zero field resistance, R0, of two MTJ devices can change over time when they are in power-on mode and exposed to a stress temperature that is significantly higher than room temperature. FIG. 2 shows that there is a general resistance increase of ˜4.5% over the original zero field resistance as well as a difference in the magnitude of the resistance increase of ˜1%. Because of this difference in resistance change between the two sensors, both the FIGS. 1B and 1C schemes will undergo some degradation of their measurement accuracy.    [1] J. Stauth, R. Dickinson, G. Forrest, and R. Vig, “Integrated Sensor,” U.S. Pat. No. 7,259,545 B2 (2007)    [2] S. Shoji, “Current Sensor,” US Patent Pub. #US 2006/0170529 A1 (2006)
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 7,440,861, Ansserlechner et al. discuss the piezoelectric effect on magnetic field sensors and use temperature compensation to mitigate the effect. U.S. Pat. No. 6,580,271 (Li et al.) discloses applying a current to induce a known magnetic filed and then using a proportioning approach to determine the unknown magnetic field. U.S. Patent Application 2007/0154740 by Yuasa et al. is an example of a patent application using normalized resistance, but only to prepare a graph.